The present invention relates to an improved apparatus for classifying or identifying noise sources, particularly vehicles, whose noise spectra are detected by a receiving device and are stored after conversion into a desired form of electrical signal and/or compared in any known classifier with earlier stored and converted noise signals corresponding to known noise sources.
In several technical fields there exists a need for the ability to classify or identify noises from technical structures which act as noise sources in order to be later able to identify, with the aid of stored noise samples and/or noise characteristics, the technical structure itself or its manufacturing series.
Such a need exists, for example, in the field of aviation, when an aircraft comes into the vicinity of an airfield over which there is a cloud cover so that it is impossible to visually observe the aircraft. Under such conditions, while the position of the aircraft can be determined by radar, its identity can only be determined with sufficient accuracy by a classification of its sound.
Similar interest exists in the field of navigation where it is the usual practice during fog or poor visibility due to haze to locate the position of strange ships by radar. However, radar screens generally display targets such as ships only in an approximated dot shape and consequently do not provide any information as to the size and type of the ship. This additional information can be obtained, however, with a device according to the present invention.
It is known that it is possible for a human being, after some practice or experience, and with the help of his memory of previously heard sounds, to recognize characteristic sounds, sound pictures or noises, to classify them and perhaps specifically identify their source. However, this method must remain imperfect due to the human fallibility so that errors cannot be excluded. The requirements for automatization therefore require instruments which can provide the corresponding information without human assistance and in a manner that can be accurately reproduced.
Methods for classifying or identifying human voice sounds are known from the published literature. For example, U.S. Pat. No. 2,891,111 describes a system for the automatic analysis of the human voice.
The literature also discloses a method for classifying vehicles, and in particular water vehicles, which method, however, requires very complicated and time-consuming preparatory work.
According to this method, the noise spectrum during every stage of movement, including the rest state, which can occur during operation for any vehicle or vehicle type which it is desired to later classify must first be recorded and stored so that the recorded noise spectra can later be compared with the noise of a vehicle to be identified. In this connection, see the paper entitled "State of the Art in Pattern Recognition", by G. Nagy, published in "Proceedings of the IEEE", Volume 56, No. 5, May, 1968, pages 832-862.
This method thus requires a large number of processing means and a large storage capacity which is not always available, and which is undesirable for economic reasons.